The article uses custom css @properties which are awesome and have 88% browser support [1].
One thing to watch out for is differences in how browsers handle setting the fallback initial-value. Chrome will use initial-value if CSS variable is undefined OR set to an invalid value. Firefox will only use initial-value if the variable is undefined. For most projects, this won't be an issue, but for a recent project, I ended up needing to use javascript to set default values in Firefox to iron out the inconsistency between browser implementations.
One thing to watch out for is differences in how browsers handle setting the fallback initial-value. Chrome will use initial-value if CSS variable is undefined OR set to an invalid value. Firefox will only use initial-value if the variable is undefined. For most projects, this won't be an issue, but for a recent project, I ended up needing to use javascript to set default values in Firefox to iron out the inconsistency between browser implementations.
[1] https://caniuse.com/?search=%40property